As shown in FIG. 1, a workpiece clamping device 10 of the prior art comprises two clamping blocks 12 and 13, which are adjusted by a bolt 11 to facilitate the clamping of a workpiece between the two clamping blocks 12 and 13. The workpiece is held securely by the clamping blocks 12 and 13 at the time when the handle 14 is pressed against the urging plate 15. If additional pressure is called for, the press plate 16 is pressed. The prior art device 10 is defective in design in that the workpiece can not be easily held, and that it is rather inconvenient to rotate the blot 11 to open up the two clamping blocks 12 and 13.
As shown in FIGS. 2 and 3, another prior art device 20 comprises a press rod 21, which must be pressed to actuate the connection rods 22 and 23 so as to force the push rod 24 to extend, thereby causing the locking piece 25 to slant to retain the slide rail 26. As a result, a workpiece is held between the two clamping blocks 28 and 29. The prior art device 20 is defective in design in that it can not clamp the workpiece with ease and speed.